new option jump-char-use-initial-char: if t, use initial char to repeat.
customize backward/forward keys. if empty, do not set it.
if jump-char-mode is on, skip setting isearch again.

After some use of jump-char (normal use and C-g to break), I found
isearch will be broken. Hope this fix it.
commentary update
fix issues caused by PR#5
add command to exit jump-char without further side effect

resolves #4